Have to do is obligation Should do is choice. Must do is compulsion. I have to do my home work... I have to do my home work, but I should clean my room first.. I must eat first, to clean my room.

Hi... I have to do is an obligation I must do is nearly the same meaning But I should do is a conditional tense so you have here a condition -" I'm watching a movie and I really should do my home work instead."

"have to" is an obligation that u urself define it "must" is an obligation too but it is not from u, it's from outside. eg. from society must is stronger than have to "should" is a kind of suggestion n it means "it's better to do s.th"

have to = must when we r talking about necessity e.g i have an exam tomorrow so i have to study or i must study must is used in obligation to give more force i must follow the traffic rules So i have no choice bcz if i didn't obey i'd be punished should is used for advice A: i'm very ill. what should i do? B: you should see a doctor.